U.S. patent number 6,320,571 [Application Number 09/152,339] was granted by the patent office on 2001-11-20 for bistable liquid crystal display device.
This patent grant is currently assigned to Ricoh Company, Ltd.. Invention is credited to Kenji Kameyama, Hiroyuki Takahashi.
United States Patent |
6,320,571 |
Takahashi , et al. |
November 20, 2001 |
Bistable liquid crystal display device
Abstract
A bistable liquid crystal display device having a plurality of
display cells which include a pair of substrates arranged
substantially in parallel, each substrate having a confronting
surface bearing transparent electrodes, and a layer of
chiral-nematic liquid crystal materials contained between the
substrates. The display device is switched between first and second
metastable states caused by relaxation from a state previously
formed by the Freedricksz transition, the first and second
metastable states corresponding to arrangements of liquid crystal
molecules gradually twisted between the substrates by 360.degree.
for the first (or T) metastable state, or by 0.degree. for the
second (or U) metastable state, respectively, and of maintaining
the selected metastable state by applied third voltages. At least
either the amplitude or width of the first voltage used to initiate
the Freedricksz transition may preferably be adjusted, to thereby
provide a liquid crystal display device of high display quality,
capable of achieving a high speed drive over a wide range of
temperatures.
Inventors: |
Takahashi; Hiroyuki (Yokohama,
JP), Kameyama; Kenji (Sagamihara, JP) |
Assignee: |
Ricoh Company, Ltd. (Tokyo,
JP)
|
Family
ID: |
17183907 |
Appl.
No.: |
09/152,339 |
Filed: |
September 14, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Sep 12, 1997 [JP] |
|
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9-248820 |
|
Current U.S.
Class: |
345/204; 345/205;
345/210; 345/211; 345/87; 345/88; 345/92; 345/95; 345/97 |
Current CPC
Class: |
G09G
3/3629 (20130101); G09G 2300/0486 (20130101); G09G
2310/06 (20130101); G09G 2320/041 (20130101) |
Current International
Class: |
G09G
3/36 (20060101); G09G 005/00 () |
Field of
Search: |
;345/204,205,210,211,87,88,91,92,95,97 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Patent Abstracts of Japan, AN 89-062655/09, JP 01051818, Feb. 28,
1989. .
Patent Abstracts of Japan, AN 5-121996, JP 6-230751, Aug. 19, 1994.
.
Patent Abstracts of Japan, AN 07199825, JP 08101371, Apr. 16, 1996.
.
Patent Abstracts of Japan, AN 07199824, JP 08313878, Nov. 29,
1996..
|
Primary Examiner: Hjerpe; Richard
Assistant Examiner: Lesperance; Jean
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A method of driving a liquid crystal display device including a
liquid crystal cell comprising the steps of:
providing the liquid crystal cell with a first voltage pulse to
initiate Freedricksz transition of said layer of said liquid
crystal material, and a second voltage used to select either said
first or second metastable state of said liquid crystal material,
and a third voltage used to maintain the selected metastable
state;
sensing a temperature at the liquid crystal display device; and
adjusting at least either an amplitude or width of said first
voltage pulse used to initiate the Freedricksz transition based on
the sensed temperature, to change a d/p value, wherein d and p
represent cell spacing of the liquid crystal cell and unstrained
pitch of the liquid crystal material, respectively.
2. A method of driving a liquid crystal display device including a
liquid crystal cell including two substrates arranged in parallel,
each substrate having a confronting surface bearing at least one
transparent electrode, an alignment film disposed over said
transparent electrode, a layer of liquid crystal material contained
between said substrates, said liquid crystal material being
chiral-nematic liquid crystal with a positive dielectric
anisotropy, a surface of said alignment film being alignment
treated with an anti-parallel alignment direction and pre-tilt
angles formed on respective alignment film surfaces by the
electrodes, said method comprising the steps of:
providing the liquid crystal cell with a first voltage pulse to
initiate Freedricksz transition of said layer of said liquid
crystal material, and a second voltage used to select either said
first or second metastable state of said liquid crystal material,
and a third voltage used to maintain the selected metastable
state;
sensing a temperature at the liquid crystal display device; and
adjusting at least either an amplitude or width of said first
voltage pulse used to initiate the Freedricksz transition based on
the sensed temperature, to change a d/p value, wherein d and p
represent cell spacing of the liquid crystal cell and unstrained
pitch of the liquid crystal material, respectively.
3. The method according to claim 2, wherein said first voltage used
to initiate the Freedricksz transition of said layer of liquid
crystal material is a pulse voltage with a magnitude equal to or
greater than a threshold voltage V.sub.th determined with respect
to an initial state and said two metastable states, and said second
voltage used to select either said first or second metastable state
of said liquid crystal material is a pulse voltage with a magnitude
which is determined with respect to a critical value V.sub.C
related to a potential difference between said metastable
states.
4. The method according to claim 3, wherein said third voltage used
to maintain the selected metastable state is a pulse voltage with a
magnitude smaller than the threshold voltage V.sub.th determined
with respect to an initial state and said two metastable
states.
5. The method according to claim 3, wherein said third voltage used
to maintain the selected metastable state is a pulse voltage with a
magnitude smaller than said critical value V.sub.C determined with
respect to a potential difference between said metastable
states.
6. The method according to claim 2, wherein said liquid crystal
cell further includes a plurality of delineated electrodes on each
substrate to serve as scan electrodes or signal electrodes, and
wherein each delineated electrode is capable of being individually
addressed in a multiplexed fashion.
7. The method according to claim 6, wherein on one of said
substrates of said liquid crystal display panel, red, green or blue
color filters are provided in a matrix corresponding to individual
pixels.
8. The method according to claim 2, further comprising the step of
arbitrarily adjusting at least either the amplitude or width of
said first voltage applied to initiate the Freedricksz
transition.
9. The method according to claim 8, further comprising the step of
sensing temperature of the liquid crystal panel, so as to adjust at
least either the amplitude or width of said first voltage used to
initiate the Freedricksz transition.
10. A method of driving a liquid crystal display device including a
liquid crystal cell including two substrates arranged in parallel,
each substrate having a confronting surface bearing at least one
transparent electrode, an alignment film disposed over said
transparent electrode, a layer of liquid crystal material contained
between said substrates, said liquid crystal material being
chiral-nematic liquid crystal with a positive dielectric
anisotropy, a surface of said alignment film being alignment
treated with an anti-parallel alignment direction and pre-tilt
angles formed on respective alignment film surfaces by the
electrodes, said method comprising the steps of:
providing the liquid crystal cell with a first voltage to initiate
Freedricksz transition of said layer of said liquid crystal
material, and a second voltage used to select either said first or
second metastable state of said liquid crystal material, and a
third voltage used to maintain the selected metastable state,
adjusting at least either an amplitude or width of said first
voltage used to initiate the Freedricksz transition,
wherein said liquid crystal material contained between said
substrates has a kinetic viscosity equal to or smaller than 17
(mm.sup.2 /sec) at 20.degree. C.
11. The method according to claim 10, wherein said liquid crystal
material contained between said substrates has a kinetic viscosity
equal to or smaller than 40 (mm.sup.2 /sec) at 0.degree. C.
12. The method according to claim 11, wherein said liquid crystal
material contained between said substrates has an anisotropy of
dielectric constant equal to or greater than 3.0.
13. A liquid crystal display device comprising:
a liquid crystal cell including two substrates arranged in
parallel, each substrate having a confronting surface bearing at
least one transparent electrode, an alignment film disposed over
said at least one transparent electrode, a layer of liquid crystal
material contained between said substrates, said liquid crystal
material being chiral-nematic liquid crystal with a positive
dielectric anisotropy, a surface of said alignment film being
alignment treated with an anti-parallel alignment direction and
pre-tilt angles formed on respective alignment film surfaces by a
molecular axis of said liquid crystal material at an initial state
being substantially equal to each other, a ratio of an unstrained
pitch to a thickness of the layer of liquid crystal material being
approximately from 1 to 3, said liquid crystal cell being switched
between first and second metastable states caused by relaxation
from a state previously formed by Freedricksz transition, the first
and second metastable states corresponding to arrangements of said
liquid crystal material gradually twisted between the substrates by
360.degree. or 0.degree., respectively;
means for applying first, second, and third voltages between the
electrodes, said first voltage being a pulse voltage used to
initiate the Freedricksz transition of said layer of said liquid
crystal material, said second voltage being used to select either
said first or second metastable state of said liquid crystal
material, and said third voltage being used to maintain the
selected metastable state; and
means for sensing a temperature at the liquid crystal display
device;
wherein at least either an amplitude or width of said first voltage
pulse used to initiate the Freedricksz transition is adjusted based
on the sensed temperature, to change a d/p value, wherein d and p
represent cell spacing of the liquid crystal cell and unstrained
pitch of the liquid crystal material, respectively.
14. The liquid crystal display device according to claim 13,
wherein said first voltage used to initiate the Freedricksz
transition of said layer of said liquid crystal material is a pulse
voltage with a magnitude equal to or greater than a threshold
voltage V.sub.th determined with respect to an initial state and
said two metastable states, and said second voltage used to select
either said first or second metastable state of said liquid crystal
material is a pulse voltage with a magnitude determined with
respect to a critical value V.sub.C related to a potential
difference between said metastable states.
15. The liquid crystal display device according to claim 14,
wherein said third voltage Used to maintain the selected metastable
state is a pulse voltage with a magnitude smaller than the
threshold voltage V.sub.th determined with respect to an initial
state and said two metastable states.
16. The liquid crystal display device according to claim 14,
wherein said third voltage used to maintain the selected metastable
state is a pulse voltage with a magnitude smaller than said
critical value V.sub.C determined with respect to a potential
difference between said metastable states.
17. The liquid crystal display device according to claim 13,
wherein said liquid crystal cell further includes a plurality of
delineated electrodes on each substrate to serve as scan electrodes
or signal electrodes, and wherein each delineated electrodes is
capable of being individually addressed in a multiplexed fashion by
said means for applying a voltage.
18. The liquid crystal display device according to claim 17,
wherein on one of said substrates of said liquid crystal display
panel, red, green or blue color filters are provided in a matrix
corresponding to individual pixels.
19. The liquid crystal display device according to claim 13,
wherein said means for applying voltage is further provided with an
additional control means to arbitrarily adjust at least either the
amplitude or width of said first voltage applied to initiate the
Freedricksz transition.
20. The liquid crystal device according to claim 19, wherein said
additional control means for applying a voltage is further provided
with automatic control means with a temperature sensor for sensing
temperature of the liquid crystal panel, so as to adjust at least
either the amplitude or width of said first voltage used to
initiate the Freedricksz transition.
21. A liquid crystal display device comprising:
a liquid crystal cell including two substrates arranged in
parallel, each substrate having a confronting surface bearing at
least one transparent electrode, an alignment film disposed over
said at least one transparent electrode, a layer of liquid crystal
material contained between said substrates, said liquid crystal
material being chiral-nematic liquid crystal with a positive
dielectric anisotropy, a surface of said alignment film being
alignment treated with an anti-parallel alignment direction and
pre-tilt angles formed on respective alignment film surfaces by a
molecular axis of said liquid crystal material at an initial state
being substantially equal to each other, a ratio of an unstrained
pitch to a thickness of the layer of liquid crystal material being
approximately from 1 to 3, said liquid crystal cell being switched
between first and second metastable states caused by relaxation
from a state previously formed by Freedricksz transition, the first
and second metastable states corresponding to arrangements of said
liquid crystal material gradually twisted between the substrates by
360.degree. or 0.degree., respectively; and
means for applying first, second, and third voltages between the
electrodes, said first voltage being used to initiate the
Freedricksz transition of said layer of said liquid crystal
material, said second voltage being used to select either said
first or second metastable state of said liquid crystal material,
and said third voltage being used to maintain the selected
metastable state,
wherein at least either an amplitude or width of said first voltage
potential used to initiate the Freedricksz transition is adjusted,
and
wherein said liquid crystal material contained between said
substrates has a kinetic viscosity equal to or smaller than 17
(mm.sup.2 /sec) at 20.degree. C.
22. The liquid crystal display device according to claim 21,
wherein said liquid crystal material contained between said
substrates has a kinetic viscosity equal to or smaller than 40
(mm.sup.2 /sec) at 0.degree. C.
23. The liquid crystal display device in accordance with claim 22,
wherein said liquid crystal material contained between said
substrates has an anisotropy of dielectric constant equal to or
greater than 3.0.
24. A liquid crystal display device comprising:
a liquid crystal cell including two substrates arranged in
parallel, each substrate having a confronting surface bearing at
least one transparent electrode, an alignment film disposed over
said transparent electrode, a layer of liquid crystal material
contained between said substrates, said liquid crystal material
being chiral-nematic liquid crystal with a positive dielectric
anisotropy, a surface of said alignment film being alignment
treated with an anti-parallel alignment direction and pre-tilt
angles formed on respective alignment film surfaces by a molecular
axis of said liquid crystal material at an initial state being
substantially equal to each other, a ratio of an unstrained pitch
to a thickness of the layer of said liquid crystal material being
approximately from 1 to 3, said liquid crystal cell being switched
between first and second metastable states caused by relaxation
from a state previously formed by Freedricksz transition, the first
and second metastable states corresponding to arrangements of said
liquid crystal molecules gradually twisted between the substrates
by 360.degree. or 0.degree., respectively;
means for applying first, second, and third voltages between the
electrodes, said first voltage being a pulse voltage used to
initiate the Freedricksz transition of said layer of said liquid
crystal material, said second voltage being used to select either
said first or second metastable state of said liquid crystal
material, and said third voltage being used to maintain the
selected mestable state; and
means for sensing a temperature at the liquid crystal display
device;
wherein at least either an amplitude or width of said first voltage
pulse used to initiate the Freedricksz transition is adjusted based
on the sensed temperature, to change a d/p value, wherein d and p
represent cell spacing of the liquid crystal cell and unstrained
pitch of the liquid crystal material, respectively.
25. A liquid crystal display device comprising:
a liquid crystal cell including two substrates arranged in
parallel, each substrate having a confronting surface bearing at
least one transparent electrode, an alignment film disposed over
said transparent electrode, a layer of liquid crystal material
contained between said substrates, said liquid crystal material
being chiral-nematic liquid crystal with a positive dielectric
anisotropy, a surface of said alignment film being alignment
treated with an anti-parallel alignment direction and pre-tilt
angles formed on respective alignment film surfaces by a molecular
axis of said liquid crystal material at an initial state being
substantially equal to each other, a ratio of an unstrained pitch
to a thickness of the layer of said liquid crystal material being
approximately from 1 to 3, said liquid crystal cell being switched
between first and second metastable states caused by relaxation
from a state previously formed by Freedricksz transition, the first
and second metastable states corresponding to arrangements of said
liquid crystal molecules gradually twisted between the substrates
by 360.degree.or 0.degree., respectively; and
means for applying first, second, and third voltages between the
electrodes, said first voltage being used to initiate the
Freedricksz transition of said layer of said liquid crystal
material, said second voltage being used to select either said
first or second metastable state of said liquid crystal material,
and said third voltage being used to maintain the selected
metastable state;
wherein at least either an amplitude or width of said first voltage
potential used to initiate the Freedricksz transition is adjusted,
and
wherein said liquid crystal material contained between said
insulating substrates has a kinetic viscosity equal to or smaller
than 17 (mm.sup.2 /sec) at 20.degree. C.
26. The liquid crystal display device according to claim 24,
wherein said liquid crystal material contained between said
substrates has a kinetic viscosity equal to or smaller than 40
(mm.sup.2 /sec) at 0.degree. C.
27. The liquid crystal display device according to claim 25,
wherein said liquid crystal material contained between said
substrates has an anisotropy of dielectric constant equal to or
greater than 3.0.
28. The liquid crystal display device according to claim 25,
wherein said cell has a plurality of delineated electrodes on each
substrate to serve as scan electrodes or signal electrodes, and
wherein each delineated electrode is capable of being individually
addressed in a multiplexed fashion by said means for applying a
voltage.
29. The liquid crystal display device according to claim 25,
wherein said means for applying a voltage is further provided with
an additional control means to arbitrarily adjust at least either
the amplitude or width of said first voltage applied to initiate
the Freedricksz transition.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to liquid crystal display
devices, and more particularly, to bistable twisted-nematic liquid
crystal devices and a method of driving such display devices.
2. Discussion of the Background
A great deal of emphasis has been placed in recent years on liquid
crystal display devices. With recognized advantages such as low
electrical power consumption and small size, liquid crystal display
devices are widely used for audio equipment, instrument panels,
office automation equipment, and other uses.
Liquid crystals, which include ordered molecules or groups of
molecules in a liquid state, are considerably useful for
fabricating these devices for switching, modulating, and otherwise
altering the characteristics of light beams. Both differences in
transmittance and in the polarizing effect of such liquid crystals
have been now utilized for fabricating these devices.
However, it would be more practical for a number of new
applications to have a liquid crystal material which has two stable
states, and which can be easily transformed from one stable state
to the other, rapidly and with a minimum expenditure of energy.
To implement a high speed drive of liquid crystal devices, a
variety of liquid crystal displays using bistable twisted-nematic
(BTN) liquid crystals have been disclosed as exemplified in
Japanese Published Patent Application No. 1-51818 and Japanese
Laid-Open Patent Applications Nos. 6-230751, 8-101371 and
8-313878.
Bistable characteristics are shown for twisted-nematic liquid
crystals in these disclosures, in which at least two pulse voltages
are applied to produce an electric field across a liquid crystal
cell. A first pulse is used to initiate the Freedricksz transition
of the liquid crystal and a second pulse is used to subsequently
relax the liquid crystal to either one of two metastable states,
thereby modulating optical transmittance or reflectivity to be
utilized as display devices.
Although principles for switching behavior of possible displays are
presented in JPA 1-51818, no description is made of driving the
displays. Also, JPA 6-230751, 8-101371 and 8-313878 propose basics
of driving simple matrix type displays. However, no description is
made of either the effects of temperature on display quality, or
methods of compensating for the effects, which is deemed important
for practical purposes for the display devices.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
novel liquid crystal display device and a method of driving the
display device, which overcome the above-noted difficulties.
It is another object of the present invention to provide a novel
liquid crystal display device of high display quality, which is
capable of achieving a high speed driving over a wide range of
temperatures for stable operations, and to provide a method of
driving a liquid crystal display device.
To achieve the forgoing and other objects, and to overcome the
drawbacks discussed above, a novel liquid crystal display device is
provided, having a liquid crystal cell in the present invention.
The liquid crystal cell includes two transparent insulating
substrates arranged substantially in parallel, each with a
confronting surface bearing at least one transparent electrode, an
alignment film disposed over the transparent electrode, and a layer
of liquid crystal materials contained between the insulating
substrates. The liquid crystal material is chiral-nematic liquid
crystal with a positive dielectric anisotropy. In addition, the
surface of the alignment film is alignment treated with an
anti-parallel alignment direction and pre-tilt angles formed on
respective alignment film surfaces by a molecular axis of the
liquid crystal material at an initial state, being equal to each
other, and having a ratio of an unstrained pitch to a thickness of
the layer of the liquid crystal material of approximately from 1 to
3.
The liquid crystal cell is capable of being switched by applying a
plurality of voltages between first and second metastable states
caused by relaxation from a state previously formed by the
Freedricksz transition, and the first and second metastable states
correspond to arrangements of the liquid crystal molecules
gradually twisted between the substrates by 360.degree. or
0.degree., respectively.
Potential voltages such as first, second, and third voltages are
applied between the electrodes of the liquid crystal cell. The
first voltage is used to initiate the Freedricksz transition of a
layer of liquid crystal molecules, the second voltage is used to
select either the first and second metastable states, and the third
voltage is used to maintain the selected metastable state.
In the liquid crystal cell of the present invention, according to
one aspect, a first voltage is a pulse voltage with a magnitude
equal to or greater than a threshold voltage which is determined
with respect to an initial state and the two metastable states, and
at least either the amplitude or width of the pulse voltage may
arbitrarily be adjusted. In addition, the third voltage used to
maintain the selected metastable state is a pulse voltage with a
magnitude smaller than a threshold voltage determined with respect
to the two metastable states.
According to another aspect of the present invention, the liquid
crystal cell has a plurality of delineated electrodes on each
substrate to serve as scan electrodes or signal electrodes, and
each of the electrodes is capable of being individually addressed
in a multiplexed fashion by a device for applying voltages.
According to still another aspect of the present invention, the
liquid crystal material contained between insulating substrates has
a kinetic viscosity of at most 17 (mm.sup.2 /sec) at 20.degree. C.,
or of at most 40 (mm.sup.2 /sec) at 0.degree. C.
According to another aspect of the present invention, liquid
crystal material contained between insulating substrates has an
anisotropy of dielectric constant of at least 3.0.
According to another aspect of the present invention, the device
for applying a voltage is further provided with a control or
automatic control having a temperature sensor, to arbitrarily
adjust at least either the amplitude or width of the first voltage
applied to initiate the Freedricksz transition.
According to another aspect of the present invention, on one of the
substrates of the liquid crystal display panel, red, green or blue
color filters may further be provided in a matrix corresponding to
individual pixels to thereby constitute a color display panel.
Methods are also disclosed for carrying out the driving of the
liquid crystal cell by applying first, second, and third voltages
between the electrodes, the first voltage being applied to initiate
the Freedricksz transition of the layer of the liquid crystal
molecules, the second voltage being applied to select either the
first and second metastable states, and the third voltage being
applied to maintain the selected metastable state. The first
voltage is a pulse voltage with a magnitude of at least a threshold
voltage determined with respect to an initial state and the two
metastable states, and either the amplitude or width of the pulse
voltage may arbitrarily be adjusted.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the present invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
FIG. 1 is a cross-sectional view of a liquid crystal display cell
in accordance with the present invention;
FIG. 2a is a graph of cell transmittance as a function of time
comparing transmittance and pulse voltages, illustrating an
application of a unipolar reset pulse and a succeeding unipolar
second pulse having an amplitude smaller than a threshold voltage
to result in a dark state;
FIG. 2b is similar to FIG. 2a except that both the reset and second
pulse are bipolar and the bipolar second pulse has an amplitude
smaller than a threshold voltage to result in a similar dark
state;
FIG. 2c is similar to FIG. 2a except that the unipolar second pulse
has an amplitude larger than the threshold voltage to result in a
bright state;
FIG. 2d is similar to FIG. 2b except that the bipolar second pulse
has an amplitude larger than the threshold voltage to result in a
similar bright state;
FIG. 3 is a graph of d/P ratio as a function of pulse amplitude of
a second pulse V.sub.2nd applied to select either T or U states
following application of a reset pulse;
FIG. 4 is a graph of d/P ratio as a function of temperature, for
which switching behavior between U and T states is achieved;
FIG. 5 is a graph of d/P ratio as a function of temperature, for
which switching behavior between U and T states is achieved in
accordance with a first embodiment of the present invention,
wherein voltage pulses are applied having a reset pulse amplitude
V.sub.R of 25 volts and a second pulse amplitude of either 2.0
volts or 4.0 volts;
FIG. 6 is a graph of d/P ratio as a function of an amplitude of a
reset pulse, for which switching behavior between U and T states is
achieved at 0.degree. C. in accordance with a first embodiment of
the present invention;
FIG. 7 is similar to FIG. 6 except that switching behavior between
U and T states is achieved at 40.degree. C.;
FIG. 8 is a block diagram of a control architecture for controlling
a liquid crystal display device in accordance with the present
invention; and
FIG. 9 is a further block diagram of a control architecture for
controlling the liquid crystal display device in accordance with
the present invention.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
In the description which follows, specific embodiments of the
present invention usefull in liquid crystal display devices,
including twisted-nematic liquid crystal layers having a bistable
character, are described.
It is understood, however, that the present invention is not
limited to these embodiments. For example, it is appreciated that
the methods of fabricating and driving the liquid crystal display
in the present invention are adaptable to any form of liquid
crystal display device. Other embodiments will be apparent to those
skilled in the art upon reading the following description.
The present invention provides a liquid crystal display device of
high display quality, capable of achieving a high speed driving
over a wider range of temperatures for stable operations, and a
method of driving a liquid crystal display device.
Referring to the Figures, a bistable twisted-nematic (BTN) liquid
crystal display device and a method of driving the display device
in accordance with the present invention will be described
hereinbelow.
The display device includes a plurality of liquid crystal display
cells, one of which is illustrated in FIG. 1. This display cell is
one exemplary embodiment of the present invention and only one of a
plurality of such cells which are included on an entire liquid
crystal display.
As illustrated in FIG. 1, a liquid crystal display cell, having a
bistable character, includes a layer 30 of liquid crystals placed
between a pair of opposing light transparent substrates 11, 12,
which are provided with respective transparent electrodes 21, 22
for applying voltages, and respective alignment films 31, 32 for
aligning liquid crystals, and respective polarizers 41, 42.
In the present invention, a liquid crystal material is preferably
used, including a chiral-nematic liquid crystal material, having a
positive dielectric anisotropy and a ratio of its unstrained pitch
to a liquid crystal cell spacing of approximately from 1 to 3.
Using the aforementioned alignment films 31,32, liquid crystal
molecules are tilt-aligned in the cell to have a slight angle of
inclination relative to the face of the substrates 11,12 and the
angles of inclination relative to each of the substrates 11,12 to
have the opposite sign. The angle of the inclination is preferably
from 2.degree. to 30.degree.. It has been found that, for
inclination angle values smaller than the above-mentioned, the
bistability of the liquid crystal material becomes less stable
resulting in a less satisfactory switching behavior, while, for
larger values of the inclination an undesired increase in viewing
angle dependence of the display quality results.
Also, in the present invention, the liquid crystal cells may
preferably have a .DELTA.nd value of about one half of a light
wavelength presently used for viewing the display, or from 0.20 to
0.35 micron and more preferably from 0.25 to 0.3 micron, wherein
.DELTA.n and d represent an optical anisotropy value of the liquid
crystal material and the thickness of the liquid crystal layer 30,
respectively.
The two polarizers 42,41 are respectively disposed on top and
bottom faces of the cell substrates 12,11. The direction of
transparency axis of one of the polarizers is arranged to have an
angle of about 45.degree., or of from 35.degree. to 55.degree.,
between the alignment direction of an underlying alignment film,
while the direction of transparency axis of the other polarizer is
arranged to be symmetric with respect to the alignment
direction.
Among device characteristics of bistable twisted-nematic (BTN)
liquid crystal devices, switching behaviors of liquid crystal cells
will be described hereinbelow.
As a plurality of voltages to be applied to drive a liquid crystal
cell of FIG. 1, driving pulse voltages include (1) a pulse voltage
to induce the Freedricksz transition of liquid crystal molecules,
which is hereinafter referred to as a "reset pulse", and (2) a
succeeding pulse voltage to select either one of metastable states
caused by the relaxation from the above-mentioned induced state,
which is referred hereinafter to as a "second pulse".
The amplitude of the reset pulse may be adjusted to be larger than
a threshold voltage (V.sub.th) necessary to cause changes from an
initial state to the metastable states and the second pulse may be
adjusted in comparison with a critical voltage (V.sub.C) necessary
to switch from one of the metastable states to the other metastable
state.
When the amplitude of the second pulse is smaller than a critical
value, a reversed rearrangement or a backward flow in the molecular
orientation from the induced state (i.e., homeotropic state) takes
place due to a rapid relaxation, and the molecules become twisted
further by 180.degree. from an initial arrangement. Namely, if the
initial twist angle is 180.degree., this rearrangement results in a
360.degree. twist angle, which is approximately the same angle as
that of the aforementioned metastable state with a 360.degree.
twist angle. This 360.degree. twisted state is hereinafter referred
to as a T-metastable state and gives rise to a dark state of the
display device of the present construction including the alignment
of the polarizers 41,42.
By contrast, when the amplitude of the second pulse is larger than
the critical value, the reversed rearrangement is suppressed and
the molecules become stable at a twist angle smaller by 180.degree.
from an initial arrangement. For the 180.degree. initial twist
angle, namely, this rearrangement results in a 0.degree. twist
angle, which is approximately the same angle as that of the other
metastable state with a 0.degree. twist angle. This 0.degree. or
untwisted state is hereinafter referred to as a U-metastable state
and gives rise to a bright state of the display device.
The changes of transmittance in a liquid display cell with a
plurality of applied voltages are illustrated in FIGS. 2a through
2d as follows.
FIG. 2a represents the case of an application of a unipolar reset
pulse and a succeeding unipolar second pulse having an amplitude
smaller than a threshold voltage to result in a dark state. FIG. 2b
is similar to FIG. 2a except that both the reset and second pulse
are bipolar and the bipolar second pulse has an amplitude smaller
than a threshold voltage to result in a similar dark state. FIG. 2c
is similar to FIG. 2a except that the unipolar second pulse has an
amplitude larger than the threshold voltage to result in a bright
state. Likewise, FIG. 2d is similar to FIG. 2b except that the
bipolar second pulse has an amplitude larger than the threshold
voltage to result in a similar bright state.
These reset and second voltages may also be unipolar as well as
bipolar. For a liquid crystal layer not to suffer from the
accumulation of electric charges, the unipolar pulses may be
applied by changing their polarity periodically, or in every other
scan line, or in every certain number of lines.
As described earlier, two metastable states are formed by the
relaxation from the initial state and the selection of either one
of these two metastable states is determined by waveforms of
applied voltages and by a ratio of d/P, wherein d and P represent
the cell spacing of a liquid crystal cell and unstrained pitch of
the liquid crystal material, respectively.
FIG. 3 is a graph of d/P ratio as a function of pulse amplitude
V.sub.2nd of the second pulse which is applied to select the
metastable states. When waveforms of the reset pulse and width
W.sub.2nd of second pulses are both fixed, the selection of
metastable states is found considerably affected by the d/P ratio
and a pulse amplitude of the second pulse V.sub.2nd.
As to the d/P ratio, in general, increased d/P values tend to give
rise to a T metastable state, while decreased d/P values result in
a U metastable state. In addition, the d/P value which divides
these metastable states changes with an amplitude of second pulses,
as illustrated in FIG. 3. Accordingly, for a d/P value which may be
determined by the liquid crystal cell presently used, a critical
amplitude is defined as the amplitude corresponding to the d/P
value mentioned just above. By applying a second pulse having an
amplitude larger than the critical amplitude, a U metastable state
results, while a T state results for an amplitude smaller than that
value.
In practical operations of the liquid crystal cell presently having
the values d and P, a d/P range can therefore be found as a range
of the aforementioned d/P ratio, for which the selection between
two metastable states can be properly achieved. This range is
hereinafter referred to as a "d/P margin". Also, in practical
operations the d/P margin can be taken as the difference between
V.sub.2nd (U) and V.sub.2nd (T), as represented by an arrow in FIG.
3.
The greater the difference between V.sub.2nd (U) and V.sub.2nd (T),
the wider the d/P margin. This is generally more advantageous for
the cell operations due to a greater allowance for scattering in
the cell spacing during manufacturing. However, for multiplexed
operations of a display device of which pixels are constituted with
scan and signal electrodes connected thereto, a voltage with a
magnitude of at least (V.sub.2nd (U)-V.sub.2nd (T))/2 has to be
supplied to the pixels even during non-selected or non-addressed
periods. When the (V.sub.2nd (U)-V.sub.2nd (T)) value becomes
unduly large, reduction in cell transmittance and display
characteristics may result. This, in turn, imposes a certain
limitation on the value of the d/P margin.
In addition, the effects of temperature on the d/P margin has to be
considered. The d/P range for the selection of U and T states
changes with temperature as illustrated in FIG. 3. The d/P value
which corresponds to the boundary between U and T states tends to
increase with decreasing temperature, while it decreases with
increasing temperature. For liquid crystal cells having a
relatively small d/P margin, therefore, the above-mentioned d/P
change with temperature may result in disadvantages such that
switching behavior between U and T states may be hindered or a
temperature range may be reduced for which the switching behavior
can be properly achieved.
In FIG. 4, the variation of d/P ratio is illustrated as a function
of temperature, for which switching behavior between U and T states
is achieved under fixed voltage conditions.
In one aspect of the method of driving the liquid crystal display
device of the present invention, at least either one of amplitude
or width of a first voltage is not fixed but is arbitrarily
adjusted to initiate the Freedricksz transition and to reset the
state of the liquid crystal molecules to an initial state.
As described earlier, although the d/P range for selecting either U
or T states changes with temperature, the present inventors have
found that a similar change occurs with the magnitude of a reset
pulse voltage (i.e., pulse amplitude or width). Namely, the d/P
value which corresponds to the boundary between U and T states
tends to increase with a decreasing reset pulse voltage (i.e.,
pulse amplitude or width), while it decreases with an increasing
reset pulse voltage.
By adjusting the magnitude of the reset pulse, accordingly, the
change of d/p value with temperature can be compensated for. More
specifically, the adjustments can be achieved such that, for an
increase in temperature (i.e., decrease in d/P value corresponding
to the above-mentioned boundary), a reset pulse voltage is
decreased to thereby increase the d/P value, while for a decrease
in temperature, a reset pulse voltage is increased.
In another aspect of the method of driving the liquid crystal
display device of the present invention, a first voltage used to
initiate the Freedricksz transition of a layer of the liquid
crystal material is a pulse voltage with a magnitude equal to or
greater than a threshold voltage (V.sub.th) which is determined
with respect to an initial state and the two metastable states, and
a second voltage used to select either the first or second
metastable state is a pulse voltage with a magnitude which is
determined with respect to a critical value (V.sub.C) which is
related to a potential difference between the metastable
states.
By such adjustment of pulse voltages, the liquid crystal layer can
be properly brought to a reset state, and is assured to
subsequently be selectively transited to either the U or T state.
These adjustments may be made through either adjusting pulse
amplitude or width. However, since the adjustment of the width may
generally cause a change in the driving frequency of the device,
adjustment of only amplitude is preferred to avoid the frequency
change.
Also, by driving the device with these voltage adjustments, a
liquid crystal display device having a larger display capacity and
a proper switching capability over a wide range of temperatures
becomes feasible. This is particularly advantageous for a display
device having a display panel which includes a plurality of
delineated electrodes on each substrate to serve as scan electrodes
or signal electrodes, and in which each of the electrodes is
capable of being individually addressed in a multiplexed fashion by
a device for applying voltages.
In still another aspect of the method of driving the liquid crystal
display device of the present invention, a third voltage V.sub.ns
applied to maintain the selected metastable state is a pulse
voltage with a magnitude smaller than a threshold voltage V.sub.th
which is determined as a non-selected voltage (V.sub.2nd
(U)-V.sub.2nd (T))/2 in comparison to the initial state and two
metastable states.
Through the adjustment of the third voltage, the metastable state
previously selected by the second pulse may be properly maintained.
For V.sub.ns values greater than V.sub.th, the selection between U
and T states can not be properly achieved, that is, the liquid
crystal layer is always set at the reset state. In addition, the
third voltage V.sub.ns applied to maintain the selected metastable
state is preferably a pulse voltage with a magnitude smaller than
the critical value V.sub.C which is a potential difference between
the metastable states. Through the adjustment of the third voltage,
the metastable state may be properly maintained.
In another aspect of the method of driving the liquid crystal
display device of the present invention, a liquid crystal material
contained between the insulating substrates preferably has a
kinetic viscosity equal to or smaller than 17 (mm.sup.2 /sec) at
20.degree. C.
The present inventors have found that (1) d/P margin is closely
related to a kinetic viscosity value of a liquid crystal material
presently used, and (2) the smaller the kinetic viscosity value,
the larger the d/P margin, while the larger the viscosity value,
the smaller the d/P margin. This is considered to be related to the
speed of the transition of the liquid crystal molecules to the
reset state in such a manner that (1) as the kinetic viscosity
value is smaller, the liquid crystal molecules approach the
homeotropic state more completely at the final stage during the
application of the reset pulse, (2) this results in larger
back-flow movements (or a greater speed of relaxation from the
reset state) succeeding the reset pulse, (3) this thus results in a
larger difference in the ordered state of liquid crystal molecules
under applied voltages V.sub.2nd (U) and V.sub.2nd (T), and (4)
this gives rise to the increase in the d/P margin.
Using a plurality of liquid crystal materials varying in kinetic
viscosity values during experimentation, a relatively large d/p
margin is found for a kinetic viscosity value equal to or less than
17 (mm.sup.2 /sec).
Furthermore, a liquid crystal material contained in the display
cell preferably has a kinetic viscosity equal to or smaller than 40
(mm.sup.2 /sec) at 0.degree. C. As described above, the d/P margin
is closely related to a kinetic viscosity value of a liquid crystal
material presently used and preferably has a kinetic viscosity
equal to or smaller than 17 (mm.sup.2 /sec) at 20.degree. C. or at
room temperature.
The kinetic viscosity value is temperature dependent, in general,
and increases with decreasing temperature. This may result in
reduced d/P margin with decreasing temperature and a shift of the
median of the aforementioned d/P range.
During experimentation at 0.degree. C. using a plurality of liquid
crystal materials varying in kinetic viscosity values of at most 17
(mm.sup.2 /sec) at 20.degree. C. and also of at most 40(mm.sup.2
/sec) at 0.degree. C., there have been found (1) a relatively small
decrease of the d/P range with decreasing temperature, and (2) a
relatively wide range of temperature in which the selection between
U and T states is properly carried out under the above-mentioned
various driving conditions of the display device.
In another aspect of the method of driving the liquid crystal
display device of the present invention, a liquid crystal material
contained in the display cell preferably has an anisotropy of
dielectric constant equal to or greater than 3.0.
The d/P margin is closely related to an anisotropy of dielectric
constant .DELTA..di-elect cons. of a liquid crystal material
presently used as well as its kinetic viscosity value described
above, and the d/P margin tends to increase with increasing
.DELTA..di-elect cons. value, while it decreases with decreasing
.DELTA..di-elect cons. latter value.
During experimentation using a plurality of liquid crystal
materials varying in .DELTA..di-elect cons. values, a relatively
large d/p margin is found for .DELTA..di-elect cons. values of
approximately equal to or less than 17 (mm.sup.2 /sec).
In another aspect of the method of driving the liquid crystal
display device of the present invention, a device for applying
voltages is further provided with a control to arbitrarily adjust
at least either one of amplitude or width of the first voltage
applied to initiate the Freedricksz transition.
As seen on liquid crystal display devices mounted on various
computer devices such as, for example, laptop personal computers
and word processors, there have been provided control knobs for an
operator to adjust display quality. In the BTN type display devices
of the present invention a similar measure is also taken for an
operator to be able to arbitrarily adjust a magnitude of reset
pulses, thereby acquiring driving methods for achieving an
excellent display quality of the liquid crystal display device,
which can be maintained over a wide range of temperatures.
Furthermore, the liquid crystal display device described just above
is further provided with an additional control capable of
automatically controlling the magnitude of a reset pulse. This
additional control is constituted of a temperature sensor for
sensing the temperature of the liquid crystal panel, and a memory
(ROM) and its control circuit for storing a plurality of V.sub.R
values which have been obtained in advance so as to properly
achieve the switching behavior of the panel at various temperatures
as illustrated in, for example, FIG. 3. These V.sub.R values are
determined at various temperatures and are subsequently programmed
by being stored in the memory to compensate the change with
temperature in the d/P boundary values which are related to the
magnitude of second pulses.
By sensing the temperature of the display panel and by supplying
voltage signals output from the additional control, a display panel
can be properly operated and can retain an excellent display
quality over a wide range of temperatures even for BTN type liquid
crystal displays without cautious adjustments by an operator.
Although the present invention has been described up to this point
primarily on methods of operating the display devices, a display
device using a bistable twisted-nematic (BTN) liquid crystal of the
present invention will be further detailed hereinbelow.
The display device includes a plurality of liquid crystal display
cells, one of which is illustrated in FIG. 1. This display cell is
one exemplary embodiment of the present invention and only one of a
plurality of such cells which are included on an entire liquid
crystal display.
Referring to FIG. 1, a liquid crystal cell includes two transparent
insulating substrates 11, 12 arranged substantially parallel, each
with a confronting surface bearing respective transparent
electrodes 21,22, a respective alignment film 31,32 disposed over
the transparent insulating electrodes 11, 12, and a layer 30 of
liquid crystal materials contained between the transparent
insulating substrates 11, 12.
In the present invention, the liquid crystal material preferably
used, including a chiral-nematic liquid crystal material, has a
positive dielectric anisotropy and a ratio of its unstrained pitch
to the liquid crystal cell spacing of from about 1 to 3.
In addition, the liquid crystal cell is constructed to be capable
of being switched between first and second metastable states caused
by relaxation from a state (or an initial state) previously formed
by the Freedricksz transition. The first and second metastable
states each correspond to the arrangement of the liquid crystal
molecules being gradually twisted between the substrates by
360.degree. for the first (or T) metastable state, and by 0.degree.
for the second (or U) metastable state, respectively.
The liquid crystal cells of the display device are further provided
with a device (see FIGS. 8 and 9) for applying various voltages at
transparent electrodes 21, 22. These voltages includes first,
second, and third voltages, wherein the first voltage is used to
initiate the Freedricksz transition of, (or to reset) the layer 30
of the liquid crystal material, the second voltage is used to
select either the first or second metastable state of the liquid
crystal material, and the third voltage is applied to maintain the
selected metastable state.
In the present invention, at least one of either amplitude or width
of the first voltage used to initiate the Freedricksz transition
may preferably be adjusted.
In FIG. 8, a block diagram of a liquid crystal display device with
its control architecture in accordance with one embodiment of the
present invention is illustrated.
In one aspect of the liquid crystal display device of the present
invention, at least either amplitude or width of a first voltage is
not fixed but is arbitrarily adjusted to initiate the Freedricksz
transition and to reset the state of the liquid crystal
molecules.
As described earlier, although a d/P range for the selection
between U and T states changes with temperature, the present
inventors have found that a similar change is found with the change
of a reset pulse voltage (i.e., pulse amplitude or width). That is,
the d/P value which corresponds to the boundary between U and T
states tends to increase with decreasing reset pulse voltage, while
it tends to decrease with increasing reset pulse voltage.
By adjusting the magnitude of the reset pulse, accordingly, the
change of d/p value with temperature can be compensated for.
More specifically, the adjustments can be achieved such that, for
the increase in temperature (i.e., decrease in d/P value
corresponding to the above-mentioned boundary), a reset pulse
voltage is decreased to thereby increase the d/P value, while for
the decrease in temperature, a reset pulse voltage increases.
The following examples are provided to further illustrate preferred
embodiments of the present invention.
EXAMPLES
A liquid crystal display device was fabricated in accordance with
the present invention. The display device includes generally a
plurality of liquid crystal display cells, one of which is
illustrated in FIG. 1. This display cell is one exemplary
embodiment of the invention and only one of a plurality of such
cells which are included on an entire liquid crystal display.
Referring to FIG. 1, a liquid crystal cell includes a pair of
transparent insulating substrates 11,12 arranged substantially in
parallel, each with a confronting surface bearing a respective
transparent electrodes 21,22, a respective alignment film 31,32
disposed over the transparent electrode, a layer 30 of liquid
crystal materials contained between the transparent insulating
substrates 11,12, and respective polarizing plates 41,42 attached
to each of outside surfaces of the substrates 11,12.
In the present invention, the liquid crystal material preferably
used, including chiral-nematic liquid crystal material, has a
positive dielectric anisotropy and a ratio of its unstrained pitch
to the liquid crystal cell spacing of approximately from 1 to
3.
Furthermore, a liquid crystal display device was provided with a
display panel, which includes a plurality of display pixels defined
by delineated stripes of transparent electrodes and arranged in a
matrix. The stripes of transparent electrodes were connected to
driving circuits to serve as scanning and signal electrodes for
driving the liquid crystal panel. By subsequently providing a back
light on the rear side of the panel to illuminate the panel, a
liquid crystal display device illustrated in FIG. 8 was
fabricated.
The liquid crystal panel was controlled by a driving device as
shown in FIG. 8. This driving device includes row 84 and column 82
driving circuits which supply driving voltages to row and column
electrodes of pixels of the liquid crystal display panel 80, and a
plurality of circuits which control the row 84 and column 82
driving circuits. The plurality of circuits included a reference
signal generator 86, a sequential scanning circuit 88, and a
voltage control circuit 89 with an external resistance 87.
Since operation methods and major characteristics of the display
device were described earlier, descriptions specifically in
accordance with several embodiments will be made hereinbelow.
Example 1
A liquid crystal display device according to one embodiment of the
present invention was fabricated as follows.
On the surface of a first transparent substrate of glass,
delineated transparent electrodes of indium tin oxide (ITO) were
formed to serve as scanning or signal electrodes of the display
device. The first substrate having the transparent electrodes was
then coated with a layer of polyimide (AL-3046 from Japan Synthetic
Rubber Co) and was subsequently alignment treated by rubbing the
surfaces of the polyimide layer in a uniform direction.
A second substrate was also provided, in a similar manner as above,
having transparent electrodes and an alignment treated polyimide
layer. In order to form a wedge shaped liquid crystal cell, the
second substrate was subsequently arranged spaced apart from the
first substrate to have a cell spacing continuously changing from
one end to the other of the cell by interposing spacers of plastic
films of different thickness therebetween, and at each opposing end
of the substrates. These first and second substrates were also
positioned such that the polyimide layers were on inner confronting
surfaces of the substrates and that directions of the alignment
made a 180.degree. angle (or anti-parallel) between the first and
second substrates.
A liquid crystal material was disposed and then sealed between the
substrates, whereby the above-mentioned wedge shaped liquid crystal
cell was constituted.
Prior to sealing these substrates, a liquid crystal material was
prepared with a nematic liquid crystal ZLI-1557 from Merck & Co
(birefringence of .DELTA.n=0.1147), mixed with a chiral nematic
liquid crystal S-811 from Merck & Co which induced a
right-handed helical structure, so as to have a predetermined
unstrained pitch (p) of 3.7 microns. It is noted for the present
material that the change of the pitch with temperature is about 2%
for 40 degrees from 0.degree. C. to 40.degree. C., which is
negligibly small for practical purposes.
The liquid crystal cell was further provided with a pair of
polarizers which were placed on the surfaces of the substrate
opposite to the surfaces thereof which contact the liquid crystal
material.
At this point, the transparent axes of the two polarizers were
positioned to be perpendicular to each other such that each of the
axes had a 45.degree. angle with respect to the direction of the
alignment treatment, being symmetric with respect to the alignment
direction.
Using the wedge shaped liquid crystal cell thus prepared, the
position of the boundary between U and T states in the cell could
be observed under various experimental conditions. From the values
of a cell spacing and the pitch (p) of a liquid crystal material,
together with the above-mentioned position of the boundary, the
range of the ratio d/p and the temperature dependence thereof was
obtained, for which switching behavior between the U and T states
was properly achieved. This switching accessible range is
hereinafter referred to as a "d/p range", as described earlier.
In the present example, voltage pulses were adjusted as follows and
applied in common throughout the measurements to a liquid crystal
cell at various temperatures.
Reset pulse width (W.sub.R): 2 msec;
2nd pulse width (W.sub.2nd): 125 .mu.sec;
Reset pulse amplitude (V.sub.R): 25 volts.
In addition to these pulses, a couple of reset pulses were further
applied individually, having amplitudes (V.sub.2nd) of either 2.0
volts or 4.0 volts at a frame frequency of 50 Hz (20
msec/frame).
When the above-mentioned position of the boundary between U and T
states in the cell was observed under various experimental
conditions, it was found that the "d/p range" changed considerably
with temperature, and results of the measurements are shown in FIG.
5.
Subsequently, another group of measurements was carried out at
0.degree. C. and 40.degree. C., with reset pulses varying in
amplitudes (V.sub.R), in place of the fixed 25 volts amplitude
previously adopted, to observe d/p ranges and its V.sub.R
dependence. The results from the measurements indicate that, by
adjusting the amplitude of the reset pulse V.sub.R, the value of
d/p range can be brought approximately to that at an arbitrary
temperature such as 20.degree. C., for example, and that the value
can be retained constant over a range of temperatures. The
variations of the d/p range with the amplitude of the reset pulse
VR were measured at 0.degree. C. and 40.degree. C. and the results
are shown in FIGS. 6 and 7, respectively.
Examples 2 through 6 and Comparative Example 1
A liquid crystal cell was fabricated in a similar manner to Example
1, with the exception that the cell spacing was adjusted to be
about 2.1 microns throughout the cell area by using silica beads
having an approximately same diameter, in place of the spacing of
the wedge shaped cells of Example 1, which changed continuously
from one end to the other.
Switching characteristics of the display cell were subsequently
examined at 20.degree. C. by applying various reset and second
pulses.
In the measurements, W.sub.R and W.sub.2nd were fixed to be 2 msec
and 125 .mu.sec, respectively, while V.sub.R and V.sub.2nd values
changed.
The values of V.sub.2nd (U) and V.sub.2nd (T) were then obtained as
threshold amplitudes to achieve the transition to U and T states,
respectively. The averages of V.sub.2nd (U) and V.sub.2nd (T)
values were calculated as critical values V.sub.C for the
transition and are shown in Table 1. Also during the measurements,
a V.sub.R value of 10 volts was intentionally adopted to supplement
data as a comparative example, which results are also included in
Table 1.
TABLE 1 V.sub.R (volts) V.sub.C (volts) COMPARATIVE EXAMPLE 1 10 --
EXAMPLE 2 15 3.7 EXAMPLE 3 20 3.3 EXAMPLE 4 25 3.0 EXAMPLE 5 30 2.7
EXAMPLE 6 35 2.5
The results in Table 1 indicate (1) in Examples 2 through 6, a
critical value V.sub.C was obtained for each V.sub.R value, and the
switching between the two metastable states U and T was able to be
accomplished by applying V.sub.2nd pulses which were appropriately
determined from the V.sub.C values such that the amplitudes of
V.sub.2nd (U) and V.sub.2nd (T) were respectively larger and
smaller than V.sub.C, and (2) in Comparative Example 1, however,
the reset pulse V.sub.R was too small to accomplish the switching
between U and T states and to thereby deduce its V.sub.C value.
Examples 7 through 10 and Comparative Example 2
Switching characteristics of liquid crystal cells were measured at
20.degree. C. by applying various reset and second pulses with
fixed values of V.sub.R of 25 volts and W.sub.2nd of 125 .mu.sec,
and with changing values of W.sub.R and V.sub.2nd. The liquid
crystal cells had the same construction as those used in the
previous Examples 2 through 6.
In these measurements, by changing the V.sub.2nd value for each of
the V.sub.R values, V.sub.C values were obtained in a similar
manner as described earlier, as critical values between the two
metastable states. Results from the measurements are shown in Table
2.
TABLE 2 W.sub.R (msec) V.sub.C (volts) COMPARATIVE EXAMPLE 2 0.25
-- EXAMPLE 7 0.5 3.9 EXAMPLE 8 1.0 3.4 EXAMPLE 9 2.0 3.0 EXAMPLE 10
4.0 2.5
The results in Table 2 indicate that (1) in Examples 7 through 10,
a critical value V.sub.C was obtained for each W.sub.R value, and
the switching between the two metastable states U and T can be
accomplished by applying W.sub.R pulses which are appropriately
determined from the V.sub.C values such that the amplitudes of
V.sub.2nd (U) and V.sub.2nd (T) are respectively larger and smaller
than V.sub.C, and (2) in Comparative Example 2, however, the reset
pulse W.sub.R was too small to securely reset the cell to thereby
deduce its V.sub.C value.
Examples 11 through 30 and Comparative Examples 3 and 4
A liquid crystal device was fabricated in a similar manner to
Examples 2 through 6. In addition, with previously disposed
delineated stripes of transparent electrodes, a plurality of pixels
were defined in a matrix to thereby constitute a liquid crystal
panel. The stripes of transparent electrodes served as scanning and
signal electrodes for driving the panel. The pixels on the panel
were then driven in a multiplex fashion by supplying scanning and
drive signals to the scanning and signal electrodes,
respectively.
The signals were selected and supplied to the electrodes having
fixed values such as W.sub.R of 2 msec, V.sub.R of 25 volts, and
W.sub.2nd (T) of 2 volts, while V.sub.2nd (U) values changed. For
each of the V.sub.2nd (U) values, a non-selected voltage V.sub.ns
=(V.sub.2nd (U)-V.sub.2nd (T))/2 was calculated, and also the
stability of the selected metastable state was observed visually.
The results from the measurements are shown in Table 3.
It may be added for the pixel matrix that (1) W.sub.C and W.sub.th
were found to be 3.0 volts and 11.0 volts, respectively, (2) the
cell matrix was brought to the reset state for the non-selected
voltage V.sub.ns larger than the 11.0 volts V.sub.th , to thereby
not be able to accomplish the selection of either U or T state, and
(3) the stability of the selected metastable state was observed
highest for the non-selected voltage V.sub.ns smaller than the 3.0
volts V.sub.C, as indicated in Table 3.
TABLE 3 (V.sub.2nd (U)- V.sub.2nd (U) V.sub.2nd (T))/2 Stability of
selected (volts) (volts) metastable state EXAMPLE 11 4 1.0
.circleincircle. EXAMPLE 12 5 1.5 .circleincircle. EXAMPLE 13 6 2.0
.circleincircle. EXAMPLE 14 7 2.5 .circleincircle. EXAMPLE 15 8 3.0
.largecircle. EXAMPLE 16 9 3.5 .largecircle. EXAMPLE 17 10 4.0
.largecircle. EXAMPLE 18 11 4.5 .largecircle. EXAMPLE 19 12 5.0
.largecircle. EXAMPLE 20 13 5.5 .DELTA. EXAMPLE 21 14 6.0 .DELTA.
EXAMPLE 22 15 6.5 .DELTA. EXAMPLE 23 16 7.0 .DELTA. EXAMPLE 24 17
7.5 .DELTA. EXAMPLE 25 18 8.0 .DELTA. EXAMPLE 26 19 8.5 .DELTA.
EXAMPLE 27 20 9.0 .DELTA. EXAMPLE 28 21 9.5 .DELTA. EXAMPLE 29 22
10.0 .DELTA. EXAMPLE 30 23 10.5 .DELTA. COMPARATIVE 24 11.0 X
EXAMPLE 3 COMPARATIVE 25 11.5 X EXAMPLE 4 .circleincircle. No
effects of V.sub.ns were observed on the stability of selected
metastable state. .largecircle. Although the selected state was
maintained, slight effects were observed on cell transmittance.
.DELTA. Although the selected state was maintained, a certain
degree of instability was observed on cell transmittance. X Reset
states only were observed.
Examples 31 through 35 and Comparative Examples 5 through 10
A plurality of wedge shaped liquid crystal cells were fabricated in
a similar manner to Example 1, with the exception that a variety of
liquid crystal materials varying in kinetic viscosity values were
disposed between the substrates.
Switching characteristics of the thus prepared liquid crystal cells
were measured at 20.degree. C. by applying various reset and second
pulses with fixed values of W.sub.R of 2 msec, W.sub.2nd of 125
.mu.sec, V.sub.R of 25 volts, and with V.sub.2nd of either 2.0
volts or 4.0 volts applied at a frame frequency of 50 hertz.
From the values of a cell spacing and the pitch (p) of a liquid
crystal material, together with the above-mentioned position of the
boundary, the aforementioned d/p margin was obtained, for which
switching behavior between the U or T states was properly observed.
The results from the measurements are shown in Table 4.
TABLE 4 Kinetic viscosity at 20.degree. C. (mm.sup.2 /sec) d/p
margin EXAMPLE 31 12 0.101 EXAMPLE 32 13 0.082 EXAMPLE 33 14 0.085
EXAMPLE 34 15 0.071 EXAMPLE 35 17 0.062 COMPARATIVE EXAMPLE 5 19
0.029 COMPARATIVE EXAMPLE 6 20 0.031 COMPARATIVE EXAMPLE 7 21 0.025
COMPARATIVE EXAMPLE 8 25 0.011 COMPARATIVE EXAMPLE 9 30 0.005
COMPARATIVE EXAMPLE 10 44 --
The results in Table 4 indicate that d/p margins of the relatively
large magnitude were obtained in Examples 31 through 35, in which
the kinetic viscosity values were equal to or less than 17
(mm.sup.2 /sec). By contrast, in Comparative Examples 5 through 10,
where the values were larger than 17 (mm.sup.2 /sec), d/p margins
were found relatively small.
Examples 36 through 40 and Comparative Examples 11 through 13
A plurality of wedge shaped liquid crystal cells were fabricated in
a similar manner to Example 1, with the exception that a variety of
liquid crystal materials were disposed between the substrates,
wherein these materials had the same 17(mm.sup.2 /sec) kinetic
viscosity value at 20.degree. C., while each had various different
kinetic viscosity values at 0.degree. C.
Switching characteristics of the thus prepared liquid crystal cells
were measured at 20.degree. C. or at 0.degree. C. by applying
various reset and second pulses with fixed values of W.sub.R of 2
msec, W.sub.2nd of 125 .mu.sec, V.sub.R of 25 volts, and with
V.sub.2nd of either 2.0 volts or 4.0 volts applied at a frame
frequency of 50 hertz.
From the values of a cell spacing and the pitch (p) of a liquid
crystal material, together with the above-mentioned position of the
boundary, there were obtained for each cell (1) the aforementioned
d/p margin, (2) the amount of the decrease in the d/p margin
between 20.degree. C. and at 0.degree. C., and (3) the shift of the
median of the d/p margin between 20.degree. C. and at 0.degree.
C.
The results from the measurements are shown in Table 5.
TABLE 5 Decrease Shift of Kinetic Kinetic in d/p median of
viscosity viscosity margin d/p margin at 20.degree. C. at 0.degree.
C. (20.degree. C. (20.degree. C. (mm.sup.2 /sec) (mm.sup.2 /sec)
.fwdarw. 0.degree. C.) .fwdarw. 0.degree. C.) EXAMPLE 36 14 35 0.01
0029 EXAMPLE 37 13 37 0.0015 0035 EXAMPLE 38 14 39 0.0018 0038
EXAMPLE 39 15 40 0.0021 0046 EXAMPLE 40 17 40 0.002 0041
COMPARATIVE 17 43 0.0049 0075 EXAMPLE 11 COMPARATIVE 15 45 0.0055
0079 EXAMPLE 12 COMPARATIVE 17 49 0.0061 0092 EXAMPLE 13
The results in Table 5 indicate that, in Examples 36 through 40, in
which the kinetic viscosity values were equal to or less than 40
(mm.sup.2 /sec), the amount of the decrease in the d/p margin and
the shift of the median of the d/p margin were both found to be
relatively small, which are advantageous for practical
purposes.
Examples 41 through 45 and Comparative Examples 14 through 16
A plurality of wedge shaped liquid crystal cells were fabricated in
a similar manner to Example 1, with the exception that a variety of
liquid crystal materials varying in anisotropy of dielectric
constant .DELTA..di-elect cons. were disposed between the
substrates.
Switching characteristics of the thus prepared liquid crystal cells
were measured at 20.degree. C. by applying various reset and second
pulses with fixed values of W.sub.R of 2 msec, W.sub.2nd of 125
.mu.sec, V.sub.R of 25 volts, and with V.sub.2nd of either 2.0
volts or 4.0 volts applied at a frame frequency of 50 hertz.
From the values of a cell spacing and the pitch (p) of a liquid
crystal material, together with the above-mentioned position of the
boundary, the aforementioned d/p margin was obtained, for which
switching behavior between the U or T states was observed. The
results from the measurements are shown in Table 6.
TABLE 6 .DELTA..epsilon. d/p margin EXAMPLE 41 8.6 0.101 EXAMPLE 42
7.4 0.094 EXAMPLE 43 5.0 0.085 EXAMPLE 44 4.6 0.073 EXAMPLE 45 3.2
0.059 COMPARATIVE EXAMPLE 14 2.7 0.027 COMPARATIVE EXAMPLE 15 1.1
0.015 COMPARATIVE EXAMPLE 16 0.24 --
The results in Table 6 indicate that d/p margins of the relatively
large magnitude were obtained in Examples 31 through 35, in which
.DELTA..di-elect cons. values were equal to or greater than
approximately 3.0. By contrast, in Comparative Examples 14 through
16, in which the values were smaller than 3.0, d/p margins were
found relatively small.
Example 46
A liquid crystal device was fabricated in a similar manner to
Examples 2 through 6. With previously disposed delineated stripes
of transparent electrodes, a plurality of pixels were defined as a
matrix to thereby constitute a liquid crystal display panel. The
stripes of transparent electrodes were connected to driving
circuits to serve as scanning and signal electrodes for driving the
liquid crystal panel.
In addition, to illuminate the panel, a back light was further
provided on the rear side of the panel, whereby a liquid crystal
display device illustrated in FIG. 8 was constructed.
The liquid crystal panel was controlled by a driving device of FIG.
8. The driving device included row 84 and column 82 driving
circuits which supplied driving voltage waveforms to row and column
electrodes of pixels of liquid crystal display panel 80 and a
plurality of circuits which controlled the row 84 and column 82
driving circuits. The plurality of circuits included a reference
signal generator 86, a sequential scanning circuit 88, and a
voltage control circuit 89.
The driving means of the invention was further provided with an
additional control of an adjusting knob (e.g., a variable external
resistance 87 in FIG. 8), which enables an operator to arbitrarily
adjust only V.sub.R values.
Switching characteristics of the thus prepared liquid crystal
device were measured by supplying reset and second pulses with 2
msec W.sub.R, 125 .mu.sec W.sub.2nd, 4 volt V.sub.2nd (U), and 2
volt V.sub.2nd (T).
Firstly, display image control signals were input to the panel at
20.degree. C. When V.sub.R values were adjusted with the control
knob such that the selection of U or T state was properly carried
out on the panel, the V.sub.R value was found approximately 24
volts.
Subsequently, the panel was brought to and retained for one hour at
40.degree. C., to confirm the selection of U or T state was not
properly carried out with the 24 volt V.sub.R value. Instead, by
adjusting the V.sub.R value with the control knob and decreasing to
approximately 19 volts, the selection of U or T state was
recovered.
Furthermore, when the panel was brought to and retained for one
hour at 0.degree. C., the panel was found not at U state but only
at T state with the 19 volts V.sub.R value. By increasing the
V.sub.R value with the control knob to approximately 33 volts, the
selection of U or T state was recovered.
Example 47
The liquid crystal display device of Example 46 was further
provided with another additional control 99 as shown in FIG. 9
which is capable of automatically controlling the magnitude of a
reset pulse. This additional control 99 includes a temperature
sensor 98 for sensing the temperature of the liquid crystal panel
96 and a memory (ROM) 97 and its control circuit 96 for storing a
plurality of V.sub.R values which were obtained in advance so as to
properly achieve the switching behavior of the liquid crystal
display panel 90 at various temperatures, whereby a liquid crystal
display device illustrated in FIG. 9 was constituted.
Switching characteristics of the thus prepared liquid crystal
device were then measured by supplying reset and second pulses with
2 msec W.sub.R, 125 .mu.sec W.sub.2nd, 4 volt V.sub.2nd (U), and 2
volt V.sub.2nd (T). V.sub.R values were selected and programmed,
considering (1) those previously obtained for the display panels in
the aforementioned embodiments, which were composed of similar
materials and had similar cell parameters, and (2) the dependence
of the d/p margin on temperature and V.sub.R values. These V.sub.R
values were subsequently stored in the memory to be utilized for
controlling the device at temperatures sensed on the display
panel.
During the measurements, display image control signals were input
to the panel of which temperature was varied between 0.degree. C.
and 40.degree. C., to examine whether the selection of U or T state
was properly carried out with using the thus V.sub.R values
programmed as above.
The results from the measurements indicate the selection of U or T
state was properly carried out in that range of temperature.
Example 48
A liquid crystal device was fabricated in a similar manner to
Example 47, with the exception that, on one of the substrates of
liquid crystal display panel 96, red, green or blue color filters
were provided in a matrix corresponding to individual pixels,
thereby constituting a color display panel.
Switching characteristics of the thus prepared liquid crystal
device were then observed by inputting display image control
signals to the thus prepared panel and supplying reset and second
pulses in similar manner to those in Example 47.
The results from the measurements indicate that the selection of U
or T state and a satisfactory color display were properly carried
out in that range of temperature.
This application is based on Japanese Patent Application No.
9-248820, filed with the Japanese Patent Office on Sep. 12, 1997,
the entire contents of which are hereby incorporated by
reference.
Additional modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the
present invention may be practiced otherwise than as specifically
described herein.
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